Luminaires with fluorescent tubular lamps are widely used in offices. In Europe, luminaires with multiple lamps in series are also used. For example, luminaires with three 60-cm tubular lamps are popular, such as troffers. In general, this type of luminaire has two lamps in series, driven by a high power ballast, and the third lamp is driven by a low power ballast.
The structure of this type of luminaire is shown in FIG. 1.
A first lamp 10 is driven by a low power ballast 12 and a dummy starter 14. Second and third lamps 16 and 18 are in series, and are driven as a single entity by a high power ballast 20. Each has its own dummy starter 17, 19.
There is an increasing demand for LED replacements for the fluorescent tubular lamps, and in particular which do not need the luminaire to be changed. There is accordingly a very large retrofit market for tubular LED lamps, for replacing traditional fluorescent tubular lamps. However it is challenging to make an LED driver design which enables series connection of the LED lamps with their associated driver.
It is easy to connect traditional fluorescent lamps in series, since the equivalent impedance of traditional fluorescent lamp is negative. This means the input voltage for each lamp can self-regulate when they are in series.
For an LED lamp, it is more challenging to connect the LEDs in series. In particular, the impedance of a closed loop driver is positive, which gives rise to positive feedback and the possibility of an unbalance of input voltage on each lamp.
For example, if two closed loop drivers are connected in series it becomes difficult to provide the required balance between the circuits, as a result of circuit tolerances. More specifically, for a closed loop circuit, the driver will try to keep the driver output stable, but the input impedance of the closed loop driver will change following the input voltage. For example R=Uin2/P, where P is the power, Uin is the voltage and R is the resistance. If P is fixed then R will change to follow Uin.
In a series connection, the increased resistance will in turn result in an increased voltage allocated to that driver. With two closed loop drivers connected in series, there is a possibility that one driver develops a larger and larger voltage than the other driver, and the imbalance becomes worse and worse until there are abnormal driving conditions. Even if the system reaches a stable steady state, there may still be an imbalance between the two drivers. This means that different lamps may have different outputs, and instability may also give rise to flicker.
A possible solution is to use open loop drivers. An open loop driver has an input impedance which behaves as a fixed resistor. Since an open loop driver has a relatively stable input impedance, it is easier to provide balancing between them. However, open loop control means the light output is unregulated during input or output disturbances, and the output is influenced significantly by tolerances of the main parameters of the LED driver.
EP2257124A1 discloses a circuit for connecting a low current lighting circuit to a dimmer. It has a switch that closes when the output of the bridge rectifier falls below 200 volts. When the switch closes, it provides a resistive load that provides holding current for the triac of the dimmer.